Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical Li− ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
Typically, during a charging event, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
Batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. Other metals, such as Zn, Mg, Na, Fe, Al, Ca, Si, and others, also have a favorable specific energy and energy density.
When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal, 3863 mAh/g. The highest theoretical capacity for which some practical cycling has been achieved for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials include BiF3 (303 mAh/g, lithiated), FeF3 (712 mAh/g, lithiated), LiOH.H2O (639 mAh/g), and others. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy; however, the theoretical specific energies are still very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
FIG. 1 depicts a chart 2 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles.
Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 4 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 4, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. FIG. 2 shows the significant advantage offered, at least theoretically, by the lithium-oxygen system, compared with the lithium-ion cells with a conventional positive-electrode materials such as LiyCoO2 or LiyNi0.80Co0.15Al0.05 O2. The use of other metals can also offer a higher specific energy than Li-ion cells with conventional positive electrodes. As is evident from the chart 4, lithium/air batteries, even allowing for packaging weight, are capable of providing a specific energy >600 Wh/kg and thus have the potential to enable driving ranges of electric vehicles of more than 300 miles without recharging, at a similar cost to typical lithium ion batteries.
An electrochemical cell 10 is depicted in FIG. 3. The cell 10 includes a negative electrode 12, a positive electrode 14, a porous separator 16, and a current collector 18. The negative electrode 12 is typically metallic lithium. The positive electrode 14 includes carbon particles such as particles 20 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 22. An electrolyte solution 24 containing a salt such at LiPF6 dissolved in an organic solvent such as dimethyl ether or CH3CN permeates both the porous separator 16 and the positive electrode 14. The LiPF6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 10 to allow a high power.
The positive electrode 12 is enclosed by a barrier 26. The barrier 26 in FIG. 3 is formed from an aluminum mesh configured to allow oxygen from an external source 28 to enter the positive electrode 14. The wetting properties of the positive electrode 14 prevent the electrolyte 24 from leaking out of the positive electrode 14. Oxygen from the external source 28 enters the positive electrode 14 through the barrier 26 while the cell 10 discharges, and oxygen exits the positive electrode 14 through the barrier 26 as the cell 10 is charged.
As an example of the reactions and technological requirements in a Metal/oxygen cell, during discharge of the lithium/oxygen cell, Li metal dissolved from the negative electrode, while at the positive electrode, Li+ ions in the electrolyte react with oxygen and electrons to form a solid Li2O2 (or Li2O) product, which may coat the conductive matrix of the positive electrode and/or fill the pores of the electrode. The solid product is thought to be electronically insulating, at least in its crystalline, bulk form. During charge of the cell, the Li2O2 (or Li2O) is oxidized to form O2, Li+ in the electrolyte, and electrons at the positive electrode, while at the negative electrode, Li+ in the electrolyte is reduced to form Li metal.
While lithium/air cells have been demonstrated in controlled laboratory environments, a number of issues remain before full commercial introduction of a lithium/air cell is viable. A number of investigations into the problems associated with Li-air batteries have been conducted as reported, for example, by Beattie, S., D. Manolescu, and S. Blair, “High-Capacity Lithium-Air Cathodes,” Journal of the Electrochemical Society, 2009. 156: p. A44, Kumar, B., et al., “A Solid-State, Rechargeable, Long Cycle Life Lithium-Air Battery,” Journal of the Electrochemical Society, 2010. 157: p. A50, Read, J., “Characterization of the lithium/oxygen organic electrolyte battery,” Journal of the Electrochemical Society, 2002. 149: p. A1190, Read, J., et al., “Oxygen transport properties of organic electrolytes and performance of lithium/oxygen battery,” Journal of the Electrochemical Society, 2003. 150: p. A1351, Yang, X. and Y. Xia, “The effect of oxygen pressures on the electrochemical profile of lithium/oxygen battery,” Journal of Solid State Electrochemistry: p. 1-6, and Ogasawara, T., et al., “Rechargeable Li2O2 Electrode for Lithium Batteries,” Journal of the American Chemical Society, 2006. 128(4): p. 1390-1393.
Nonetheless, several challenges remain to be addressed for lithium-air batteries. These challenges include limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture and other potentially harmful components of air, designing a system that achieves acceptable specific energy and specific power levels, reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), and improving the number of cycles over which the system can be cycled reversibly.
The limit of round trip efficiency occurs due to an apparent voltage hysteresis as depicted in FIG. 4. In FIG. 4, the discharge voltage 40 (approximately 2.5 to 3 V vs. Li/Li+) is much lower than the charge voltage 42 (approximately 4 to 4.5 V vs. Li/Li+). The equilibrium voltage 44 (or open-circuit potential) of the lithium/air system is approximately 3 V. Hence, the voltage hysteresis is not only large, but also very asymmetric.
The large over-potential during charge may be due to a number of causes. For example, reaction between the Li2O2 and the conducting matrix 22 may form an insulating film between the two materials. Additionally, there may be poor contact between the solid discharge products Li2O2 or Li2O and the electronically conducting matrix 22 of the positive electrode 14. Poor contact may result from oxidation of the discharge product directly adjacent to the conducting matrix 22 during charge, leaving a gap between the solid discharge product and the matrix 22.
Another mechanism resulting in poor contact between the solid discharge product and the matrix 22 is complete disconnection of the solid discharge product from the conducting matrix 22. Complete disconnection of the solid discharge product from the conducting matrix 22 may result from fracturing, flaking, or movement of solid discharge product particles due to mechanical stresses that are generated during charge/discharge of the cell. Complete disconnection may contribute to the capacity decay observed for most lithium/air cells. By way of example, FIG. 5 depicts the discharge capacity of a typical Li/air cell over a period of charge/discharge cycles.
What is needed is a cell which decreases the charging voltage of the battery, while still allowing for high rates of discharge (i.e., high power). A further need exists for a battery with increased efficiency. A battery with decreased charging time requirements would also be beneficial.